Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which includes an illumination device and a projection lens. The image of a mask (=reticle) illuminated via the illumination device is in this case projected via the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
To increase the image position accuracy and image quality (both along the optical axis, or in the light propagation direction, and in the lateral direction, or perpendicular to the optical axis or light propagation direction), it is known in particular to configure one or more of the optical elements (e.g., mirrors) in the optical system as adaptive or actuable elements.
Merely by way of example, it is known to configure one or more mirrors in the optical system with an actuator layer made from a piezoelectric material, wherein an electrical field having locally varying strength is produced across the piezoelectric layer with a consequence that the reflection layer system of the adaptive mirror deforms together with a local deformation of the piezoelectric layer. Consequently, (possibly also temporally varying) imaging aberrations can be at least partially corrected by way of suitably controlling the electrodes. However, the deformation of the reflection layer system of a mirror or actuation of another optical element, such as a lens element, can also be used in general to further optimize the microlithographic imaging process.
In order to allow the setting of the surface curvature of the relevant adaptive optical element in practice as precisely as possible in temporally stable fashion, various approaches are known, wherein in principle a distinction is made between model-based open-loop control of the respective actuators of the adaptive element and closed-loop control of the actuators based on measurement data.
In a known approach for realizing the model-based open-loop control of the actuators, a model employing explicit knowledge or information relating to the construction and typical material properties of the relevant element is created and used to calculate the respectively attained surface curvature, wherein controlling of the actuators is effected without knowledge of the actually attained surface curvature, and in particular without a corresponding sensor system or without a closed-loop control system.
In such an open-loop control based on an explicit model, however, in practice—as will be explained in more detail below—it may arise that the validity of an explicit model on which the open-loop control is based is limited when specific “unique” properties of the concrete optical system (for example a specific imaging behavior owing to individual manufacturing errors) are present, wherein an appropriate correction or model adaptation can prove to be extremely complex in view of the large number of influencing parameters that may need to be taken into account in this case. As concerns the closed-loop control of the actuators based on measurement data (for example measurements of the actually attained surface curvature of the respective element) that is likewise conceivable as an alternative to the previously described model-based open-loop control, the attainable exactness of the surface curvature can indeed be improved in this respect, but this can entail inter alia an increase in constructive complexity in view of the optical measurement method that is used therefor.
Reference is made merely by way of example to DE 10 2011 005 940 A1.